Russian President Vladimir Putin has arrived in India on his two-day visit and the main focus during the meeting between the two nations will be on the S-400 Triumf air defence missile systems agreement.S-400 is a powerful air defence system that can take down aircraft in the sky from the surface itself and is intended to act as a shield against incoming ballistic missiles. It is known as Russia's most advanced long-range surface-to-air missile defence system. An upgraded version of the S-300 systems, the S-400 missile system, manufactured by Almaz-Antey, has been in service in Russia since 2007.